1. Field of the Disclosure
The disclosure generally relates to process control systems and specifically to heat recovery maximizers for oil refineries having multiple parallel heat exchangers.
2. Related Technology
Process control systems, like those used in chemical, petroleum or other processes, typically include one or more centralized or decentralized process controllers communicatively coupled to at least one host or operator workstation. The process controllers are also typically coupled to process control and instrumentation devices such as, for example, field devices, via analog, digital or combined analog/digital buses. Field devices, which may be valves, valve positioners, switches, transmitters, and sensors (e.g., temperature, pressure, and flow rate sensors), are located within the process plant environment and perform functions within the process such as opening or closing valves, measuring process parameters, increasing or decreasing fluid flow, etc. Smart field devices such as field devices conforming to the well-known FOUNDATION™ Fieldbus (hereinafter “Fieldbus”) protocol or the Highway Addressable Remote Transmitter (HART®) protocol may also perform control calculations, alarming functions, and other control functions commonly implemented within the process controller.
The process controllers, which are typically located within the process plant environment, receive signals indicative of process measurements or process variables made by or associated with the field devices and/or other information pertaining to the field devices, and execute controller applications. The controller applications implement, for example, different control modules that make process control decisions, generate control signals based on the received information, and coordinate with the control modules or blocks being performed in the field devices such as HART® and Fieldbus field devices. The control modules in the process controllers send the control signals over the communication lines or signal paths to the field devices to thereby control the operation of the process.
Information from the field devices and the process controllers is typically made available to one or more other hardware devices such as operator workstations, maintenance workstations, personal computers, handheld devices, data historians, report generators, centralized databases, etc., to enable an operator or a maintenance person to perform desired functions with respect to the process such as, for example, changing settings of the process control routine, modifying the operation of the control modules within the process controllers or the smart field devices, viewing the current state of the process or of particular devices within the process plant, viewing alarms generated by field devices and process controllers, simulating the operation of the process for the purpose of training personnel or testing the process control software, and diagnosing problems or hardware failures within the process plant.
While a typical process plant has many process control and instrumentation devices such as valves, transmitters, sensors, etc. connected to one or more process controllers, there are many other supporting devices that are also necessary for or related to process operation. These additional devices include, for example, power supply equipment, power generation and distribution equipment, rotating equipment such as turbines, motors, etc., which are located at numerous places in a typical plant. While this additional equipment does not necessarily create or use process variables and, in many instances, is not controlled or even coupled to a process controller for the purpose of affecting the process operation, this equipment is nevertheless important to, and ultimately necessary for proper operation of the process.
Process control of energy management is a vital concern throughout the oil refinery industry, as well as other process systems. Typical oil refineries include some sort of distillation system. In the distillation system, crude oil is transported to a heater which heats the crude oil prior to distillation. As a result, crude oil refineries require large energy inputs as the crude oil is heated before being distilled. Heating the crude oil is necessary because hot crude oil separates more easily into its distilled components. Moreover, heating the crude oil increases efficiency and reduces fuel consumption of many refining processes. Because the crude oil is heated, the distilled products contain sensible heat from the distillation process. Sensible heat is the heat transferred to or from the product stream when there is a temperature change (either an increase or decrease) in the stream. This sensible heat is potentially reclaimable energy that, if reclaimed and reused, could boost the efficiency of the distillation process. Thus, most oil refineries include some sort of a heat recovery system, for example heat exchangers. Heat exchangers transfer some of the sensible heat from the distilled products to the crude oil stream prior to distillation. In doing so, the heat exchanger reduces the amount of fuel needed to preheat the crude oil to a predetermined temperature.
Due to the size and complexity of most modern oil refineries, a single heat exchanger is not sufficient to accomplish full preheating of the crude oil. Thus, most modern oil refineries include a network of heat exchangers that heat different streams of crude oil. These networks of heat exchangers, however, do not maximize or optimize heat recovery from the distilled products, because over time the heat exchangers operate at different levels of heat transfer efficiency due to the buildup of fouling fluids on heat exchange surfaces.
Moreover, most modern oil refineries process varying qualities of crude oil and have varying demands for the refined products. The buildup of fouling fluids, varying quality of crude oil, and varying demand for refined products all lower the efficiency of known heat exchanger systems. Alternatively, the heat exchangers may use hot process fluid streams that must be cooled, as part of a secondary process, to preheat the crude oil thus further increasing efficiency. Some process design technologies are used to specify the heat exchangers that will maximize heat recover in accordance with the refinery design criteria. After the refinery is built, the process control system must generally optimize the refinery performance within given design and economic constraints. In some cases, the crude feed heat recovery optimizer must be able to maximize heat recover for a specified crude charge rate under varying conditions. The economics of the optimizer must often be reasonable, because many refineries are relatively small and may not be able to justify a large number of instruments and control valves. In addition, given the variety of process control systems among various refineries, the optimizer should be easy to implement.
Many process plants, such as oil refineries, include equipment monitoring and diagnostic applications such as, for example, the Machinery Health® application provided by CSI Systems, or any other known applications used to monitor, diagnose, and optimize the operating state of various rotating equipment. Maintenance personnel usually use these applications to maintain and oversee the performance of rotating equipment in the plant, to determine problems with the rotating equipment, and to determine when and if the rotating equipment must be repaired or replaced. Similarly, many process plants include power control and diagnostic applications such as those provided by, for example, the Liebert and ASCO companies, to control and maintain the power generation and distribution equipment. It is also known to run control optimization applications such as, for example, real-time optimizers (RTO+), within a process plant to optimize the control activities of the process plant. Such optimization applications typically use complex algorithms and/or models of the process plant to predict how inputs may be changed to optimize operation of the process plant with respect to some desired optimization variable such as, for example, profit.
Most known heat exchange optimizer systems use a search routine to find an optimum blended temperature after the crude streams are rejoined. With known systems, small changes in the efficiency of one heat exchanger may not result in a large change in the blended temperature and thus, known systems cannot account for these small changes in efficiency. However, small changes in efficiency may lead to large losses over time.
These and other diagnostic and optimization applications are typically implemented on a system-wide basis in one or more of the operator or maintenance workstations, and may provide preconfigured displays to the operator or maintenance personnel regarding the operating state of the process plant, or the devices and equipment within the process plant. Typical displays include alarming displays that receive alarms generated by the process controllers or other devices within the process plant, control displays indicating the operating state of the process controllers and other devices within the process plant, maintenance displays indicating the operating state of the devices within the process plant, etc. Likewise, these and other diagnostic applications may enable an operator or a maintenance person to retune a control loop or to reset other control parameters, to run a test on one or more field devices to determine the current status of those field devices, or to calibrate field devices or other equipment.